Integrated CMOS porous sensor

ABSTRACT

A single chip wireless sensor ( 1 ) comprises a microcontroller ( 2 ) connected by a transmit/receive interface ( 3 ) to a wireless antenna ( 4 ). The microcontroller ( 2 ) is also connected to an 8 kB RAM ( 5 ), a USB interface ( 6 ), an RS232 interface ( 8 ), 64 kB flash memory ( 9 ), and a 32 kHz crystal ( 10 ). The device ( 1 ) senses humidity and temperature, and a humidity sensor ( 11 ) is connected by an 18 bit ΣΔ A-to-D converter ( 12 ) to the microcontroller ( 2 ) and a temperature sensor ( 13 ) is connected by a 12 bit SAR A-to-D converter ( 14 ) to the microcontroller ( 2 ). The device ( 1 ) is an integrated chip manufactured in a single process in which both the electronics and sensor components are manufactured using standard CMOS processing techniques, applied to achieve both electronic and sensing components in an integrated process.

INTRODUCTION

1. Field of the Invention

The invention relates to electronic sensors.

2. Prior Art Discussion

One of the main driving forces in the electronics industry is the desireto achieve greater integration of functionality so that production ismore automated and per-unit cost reduced. An added benefit is, ofcourse, decreased size and thus higher circuit density. Mostimportantly, for battery applications, higher integration generallyresults in lower power, due to reduced parasitic capacitances.

However in the field of sensors, and in particular wireless sensors,greater integration has been slow because of the difficultiesencountered in integration of microcontroller, A-to-D converter (ADC),memory, RF transceiver, and sensor elements in the one integrated sensordevice. These difficulties have arisen because of incompatibilities ofmaterials processing for the various elements. For example, sensorelements have conventionally been manufactured on ceramic or glasssubstrates and can not be easily integrated on silicon. RF transceivershave typically been made from bipolar transistors, which are difficultto integrate with other technologies such as CMOS. Also, many CMOShigh-resolution ADCs are made using poly-poly capacitors, which sufferfrom substrate parasitics, strain, and mis-match effects. Also, thealuminium metallisation used in IC processing is prone to corrosion,thus limiting usefulness for some types of sensor applications.

U.S. Pat. No. 6,724,612 and U.S. Pat. No. 6,690,569 describe sensordevices having both electronic and sensing components, the latter beingcapacitive electrodes. However, the electrodes require platinum or goldcoating, and deposition of a polymer as a moisture-sensing dielectric.This processing is not amenable to high-volume semiconductor processing.

The invention addresses these issues.

SUMMARY OF THE INVENTION

According to the invention, there is provided an integrated sensordevice comprising:

-   -   MOS circuits in a semiconductor substrate,    -   interconnect levels with interconnect conductors and insulating        dielectric, said levels being over the substrate and        interconnecting the MOS circuits,    -   the interconnect levels incorporating a sensor having electrodes        embedded in the interconnect dielectric, and    -   the MOS circuits including a processor for processing signals        from the sensor electrodes.

In one embodiment, the sensor comprises a porous oxide for ingress of agas or humidity being sensed.

In another embodiment, the porous oxide is carbon-doped SiO₂.

In a further embodiment, the sensor is a capactive sensor.

In one embodiment, the sensor comprises a passivation layer over thesensor electrodes.

In another embodiment, the porous oxide is deposited on the passivationlayer, and the MOS circuits detect changes in a fringe field between theelectrodes.

In a further embodiment, comprises etch stop layers between theinterconnect levels, and the passivation layer is of the samecomposition as the etch stop material.

In one embodiment, the passivation layer is of Si₃N₄ composition.

In another embodiment, the passivation layer is recessed over thesensing electrodes.

In a further embodiment, there is a porous oxide film in the recess.

In one embodiment, the porous oxide is between the electrodes and isexposed.

In another embodiment, the MOS circuits are directly beneath the sensorin a vertical dimension.

In a further embodiment, the MOS circuits include a temperature sensor.

In one embodiment, the temperature sensor comprises a PNP transistor.

In another embodiment, the MOS circuits include a microcontroller forprocessing both gas or humidity signals from the gas or humidity sensorand temperature signals from the temperature sensor to provide anenhanced output.

In a further embodiment, the enhanced output is temperature-correctedgas or humidity readings.

In one embodiment, the sensor comprises polyimide deposited over thesensor electrodes.

In another embodiment, the MOS circuits include an A-to-D converterconnected between the sensor electrodes and the processor.

In a further embodiment, the A-to-D converter comprises an array ofdummy capacitors with a constant topography surrounding active A-to-Dconverter capacitors.

In one embodiment, further comprises a light emitting diode.

In another embodiment, said diode is formed in a deep trench to a lowerinterconnect level laterally of the sensor electrodes.

In a further embodiment, the device comprises a photo-detector diode.

In one embodiment, said diode is in a deep trench in a lowerinterconnect level laterally of the sensor electrodes.

In another embodiment, the MOS circuits include a wireless transceiver.

In a further embodiment, the wireless transceiver is for communicationwith other nodes in a network, and it comprises a means for switchingchannel frequency according to a low frequency channel switching schemeupon detection of interference.

In one embodiment, an interconnect level includes a low noise amplifier.

In another embodiment, the low noise amplifier comprises a strainedsilicon region beneath a conductor.

In a further embodiment, the strained silicon is in a fifth or sixthinterconnect level above the substrate.

In one embodiment, the sensor comprises a detecting element connectedbetween pads on an upper surface of the device.

In another embodiment, the element is a gas-sensing thin film.

In a further embodiment, the element is of zinc oxide composition.

In one embodiment, said element detects sound and the MOS circuitscomprise an audio processor for processing signals from the elements.

In another aspect of the invention, there is provided a method ofproducing a sensor device of any of the above embodiments, the methodcomprising the steps of:

-   -   fabricating the MOS circuits in the substrate,    -   fabricating the interconnect levels in successive fabrication        cycles according to interconnect design to interconnect the MOS        circuits, and    -   fabricating the sensor electrodes and dielectric in a final        interconnect level.

In one embodiment, the method comprises the further step of depositing apassivation layer over the top interconnect level.

In another embodiment, the method comprises the steps of depositing anetch stop layer over each layer of dielectric in the interconnectlevels, and depositing etch stop material over the top interconnectlevel dielectric to provide a passivation layer.

In a further embodiment, porous oxide is provided as a dielectric inlower interconnect levels and regular oxide is used as a dielectric inupper interconnect levels.

In one embodiment, a strained low noise amplifier is deposited in anupper interconnect level, said amplifier comprising a strained siliconregion.

DETAILED DESCRIPTION OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawings in which:—

FIG. 1 is a block diagram of a single-chip wireless sensor device of theinvention;

FIG. 2 is a flow diagram illustrating a process for producing thedevice;

FIG. 3( a) is a cross-sectional view of the device, FIG. 3( b) is a planview of sensing electrodes; and FIG. 3( c) is a diagram showing extentof a fringe field between the electrodes;

FIG. 4 is a schematic of an A-to-D converter of the device;

FIG. 5 is a diagram showing a sensor component of an alternativeembodiment;

FIGS. 6 and 7 are cross-sectional views of alternative sensorcomponents;

FIG. 8 is a diagram of a potting arrangement for final packaging;

FIG. 9 is a circuit diagram of a 12-bit SAR A-to-D converter of thesensor device;

FIG. 10 is a layout view of the capacitor array for the SAR converter;

FIG. 11 is a block diagram of a microcontroller of the device;

FIG. 12 is cross-sectional diagram showing a sub-surface current flowpath in a strained-silicon transistor of the device;

FIG. 13 is a diagram illustrating frequency selection of a wirelesstransceiver;

FIG. 14 shows a communication scheme for a device of the invention.

FIG. 15 is a cross-sectional diagram of a gas sensing device;

FIG. 16 is a schematic cross-sectional diagram of an audio sensor; and

FIG. 17 is a cross-sectional view of an LED and photo-diode of a deviceof one embodiment

DESCRIPTION OF THE EMBODIMENTS

Gas/humidity Sensor Embodiment Referring to FIG. 1 a single chipwireless sensor 1 comprises a microcontroller 2 connected by atransmit/receive interface 3 to a wireless antenna 4. Themicrocontroller 2 is also connected to an 8 kB RAM 5, a USB interface 6,an RS232 interface 8, 64 kB flash memory 9, and a 32 kHz crystal 10. Inthis embodiment, the device 1 senses humidity and temperature, and ahumidity sensor 11 is connected by an 18 bit ΣΔ A-to-D converter 12 tothe microcontroller 2 and a temperature sensor 13 is connected by a 12bit SAR A-to-D converter 14 to the microcontroller 2.

The device 1 is a single integrated chip manufactured in a singleprocess in which both the electronics and sensor components aremanufactured using standard CMOS processing techniques, applied toachieve both electronic and sensing components in an integrated process.

The manufacturing process 20 is now described in more detail referringto FIGS. 2, 3(a), 3(b) and 3(c), and the steps are 21 to 27 inclusive.

21, Front End Processing

A substrate 41 of silicon is processed with CMOS wells, isolationoxidation, poly-silicon, and implants to form MOS components, as is wellknown in CMOS processing. Also, in the substrate a temperature-sensitivePNP transistor is formed to provide the sensor 13.

22, Lower Interconnect and Dielectric Deposition

First, second, and third interconnect levels 42 are formed. Thisinvolves three cycles of chemical vapour deposition (CVD) deposition ofa porous low-K silicon dioxide dielectric 42(a), and etching and copperplating operations to provide interconnect tracks 42(b). Each cyclefinishes in deposition of an etch stop layer 42(c) for limiting theextent of etching in the next cycle. The etch stop material is siliconnitride Si₃N₄. The silicon dioxide, the interconnect metal, and the etchstop of each cycle forms a first interconnect three-level stack 42. Theuse of a low-K dielectric allows low capacitance for faster signaltransfer between components.

23, Upper Interconnect and CVD Dielectric Deposition

Fourth and fifth interconnect levels 43 are formed. There are a furthertwo cycles of dielectric deposition and metal interconnect plating.However, in these two cycles the dielectric is “regular” SiO₂(non-porous) 43(a) for better structural strength, to counteract theweaker mechanical strength of the porous dielectric in the lower levels42. Again, these cycles involve standard CMOS techniques.

The fifth level includes a heating element 43(b) with an internaltemperature monitor for instantaneous heating and purging of thehumidity sensor 11 with immediate temperature monitoring. Also, as partof developing the fourth and fifth levels, the process adds a thin metalplate for a capacitor top metal (CTM) with a thin layer (0.04 μm) SiO₂dielectric between them to form mixed signal metal-insulator-metal (MIM)capacitors for both of the A-to-D converters.

24, CVD Deposition of SiO₂, Sensing Level

An interconnect/sensing layer 44 is formed. This is simply a nextiteration or cycle following from the previous interconnect and platingcycles and indeed the dielectric is the same as for the immediatelypreceding cycles, “regular” SiO₂. However as an integral part of platingthe top interconnect layer 44 humidity-sensing capacitive interdigitatedfingers (electrodes) 45 and reference capactive interdigitated fingers(electrodes) 46 are formed. The size and spacing of the fingers ischosen to suit the application. In this embodiment the fingers 45 and 46have a spacing of 0.5 μm. The arrangement is shown more clearly in FIG.3( b). Using oxide permittivity Kox of 3.9, this results in capacitanceof:

${Cox} = {\frac{k_{ox}e_{0}}{Tox} = {\frac{3.9 \star {8.85 \times 10^{- 12}}}{0.5 \times 10^{- 6}} = {{0.000069\mspace{14mu} F\text{/}m^{2}} = {0.069f\mspace{14mu} F\text{/}{\mu m}^{2}}}}}$

Each actual capacitive structure is about the size of a bond pad,allowing each finger to have a total length of 4000 μm. For a metalthickness of 1 μm this gives a sensor capacitance of 0.276 pF. However,the capacitance between two closely-spaced narrow conductors can beabout 10% to 30% greater than the simple parallel plate calculatedvalue, due to fringing components.

25, Deposition of Si₃N₄ Passivation Layer

A passivation layer 48 is deposited by CVD in a manner similar to thatof the conventional etch stop layers as it is also of Si₃N₄. Thepassivation layer 48 is, however, approximately 3-5 μm thick to offerphysical protection and a moisture barrier for the device 1.

26, Etch Passivation over Sensing Electrodes

That part of the passivation layer 48 over the sensing electrodes 45 isetched to a depth of 90% to leave a thin Si₃N₄ layer 48(a) ofapproximately 0.1 μm depth over the sensing electrodes.

27, CVD Deposition of Porous Oxide

The same material as is used as a dielectric in the first three levelsis now deposited by CVD in the recess formed in step 26. This is amoisture-sensing film 49 having a large specific area. Ingress of gas ormoisture causes a change in permittivity of the porous dielectric. Thiscauses a change in capacitance of the underlying sensing electrodes 45.

It will be appreciated from the above that standard Deep-Sub-Micron CMOSprocessing techniques are used, thus achieving fully integratedproduction. The sensor is made simultaneously with the rest of the chip,using the same dielectric and interconnect metal layers. This ‘standardCMOS’ method is very advantageous to the high-volume manufacturabilityof this sensor 1.

This approach has not apparently been attempted heretofore because ofthe perception that such a sensor would require polymers, and gold orplatinum plating and/or other non-standard materials which would beregarded as contaminants in a modern CMOS wafer fabrication plant.Developments in SiO₂-based compositions to achieve reduced capacitancebreak up the internal lattice structure. This makes them porous andamenable to moisture or gas penetration. Also, the silicon nitride(Si₃N₄) of CMOS processing to achieve an etch stop layer is used in thesensor architecture to act as a barrier layer to protect the integrateddevice. In the above embodiment the Si₃N₄ layer is over the sensingcomponent and it acts as a barrier to ingress of the moisture beingsensed as such penetration may corrode the electrodes in high humidityenvironments. The sensing is therefore based on use of the springeffect, as set out below.

Use of Device 1

In use, moisture ingresses into the film 49 so that it affects itsdielectric constant, and therefore the fringe field, between the sensingfingers 45. This is illustrated by the lines 55 in FIG. 3( c). Thisoccurs even though the moisture is blocked by the thin part 48(a) of thelayer 48 from accessing the spaces between the sensing fingers 45.

The sensor 1 relies on this fringe component 55 of the field between theelectrodes. For the 4000 μm, 0.27 pF structure described, the fringecomponent is about 25 to 50 fF. Because of the close proximity of the18-bit ΣΔ A-to-D converter (immediately beneath the sensor) very smallcapacitance changes are detectable, even in the fringe field. Thisconverter is shown in FIG. 4 in which the sensing fingers 45 are Cs andthe reference fingers 46 are Cr. These capacitances form thedifferential front-end of a second-order over-sampled sigma-deltamodulator, illustrating the level of integration between the sensor andconverter components. Vr and Vs provide scale and offset compensation.Very high resolution is achieved by trading off the number of samplesper second and over-sampling ratio using the decimation filters.

Referring to FIG. 5, in this embodiment, porous material 50 is deposited(or printed) on top of passivation 51, eliminating extra etching steps.However, if the passivation thickness is about 3 μm for example, thenthe spacing of the sensor capacitor fingers 45 must be increased toabout 5 μm or more in order that the fringe capacitance component stillrepresents a measurable ratio of the total capacitance. For the 4000 μmsensor structure, total capacitance is now reduced to about 27 fF, withthe variable fringing component now being in the region of 3 to 5 fF.Humidity variations of 1% or 2% now produce capacitance variations ofless than a femtoFarad—still detectable by the highly over-sampleddifferential sigma-delta high-resolution converter. 18 bits ofresolution also provides a very large dynamic range, enabling theconverter to easily cope with the highly variable and non-linearcapacitance-versus-humidity characteristics of different oxides anddifferent pore sizes from wafer to wafer and lot to lot.

Referring to FIG. 6, in this embodiment, standard CMOS processing isused, with no extra processing steps required. Polyimide is often usedas a ‘stress relief’ coating layer on silicon chips. The polyimideplacement is usually determined by a slightly oversized version of thebond-pad mask. In this example, the polyimide mask includes an extraopening to eliminate polyamide 60 from over the reference capacitor.Since polyimide is porous, the portion over the sensing capacitor nowexperiences a minute change in capacitance versus humidity.

Referring to FIG. 7, in this embodiment porous low-K oxide dielectric isused in all interconnect levels of the device, so the sensor device hasa porous low-K dielectric 70 between capacitive interdigitated fingers71. By placing a ‘dummy’ bond pad passivation opening over the sensorstructure, the surface 72 above the sensing fingers 71 is exposed foringress of moisture into the dielectric between the fingers during thebond-pad etch. This leaves passivation 73 over the full area except thesensing capacitive fingers 71. This embodiment has the advantage ofusing the standard CMOS process with no extra masks required. However,it allows access by the moisture to the capacitive fingers 71. However,for many applications this is not a problem, for example a low-humidityoffice environment where the sensor only experiences a few millivoltsapplied for a few milliseconds once every few minutes.

FIG. 8 shows a simple potting arrangement for enclosing the single-chipwireless sensor. The sensor 1 is bonded to a battery 80 by conductiveadhesive 81 and there is encapsulation 82. A former is used to keep thearea over the sensing component clear. All other areas are enclosed bythe encapsulant 82, which affords physical protection, as well asprotection of the chip and battery terminals from corrosion orelectrolytic degradation if exposed continuously to high moistureenvironments. No metal is exposed anywhere, except for an RF antennawire 83.

Alternatively, there may be no encapsulation if physical protection isless important and/or if response time to temperature variations is moreimportant.

Temperature Sensors

In addition to the metal heater temperature sensor 43(b) describedabove, a substrate PNP temperature sensor 13 is also developed as anintegral part of the substrate 41, as shown in FIG. 3( a). This relieson the well-known −2.2 mV/C Vbe characteristic of the base emitterjunction. By having a combination of humidity and temperature sensors inthe one device, there can be calculation of an enhanced reading by themicrocontroller, namely dew point. These, together with themicrocontroller 2 and the flash memory 9 allow use of look-up tables forscaling and calibration, to achieve accuracy to within 0.5° C.

Referring to FIG. 9, the 12-bit SAR converter 14 is shown. This measuresthe Vbe voltage of the PNP, or the temperature-dependent resistance ofthe metal heater monitor in a bridge configuration as shown. Theconverter achieves 12 bit resolution without any calibration circuits,as follows. Referring to FIG. 10 the capacitor array for the converter14 is in the center of the level, and it is surrounded by eight similardummy arrays 90 to ensure constant topography and excellent matching ofthe key array capacitors in the converter 14. The array is segmentedinto 7 upper bits and a 5-bit sub-DAC via coupling capacitor Cc. This,together with a small unit capacitor size of 7×7 μm, keeps the entirearray capacitance (Cs) at around 8 pF, small enough that it can bedriven efficiently with an on-chip buffer amplifier as shown, and alsosmall enough that global mismatches due to gradients in oxide thicknessor other process parameters are minimized. At a sampling frequency of100 KHz, the kT/C noise figure is 140 nV, well below the 12-bit LSB sizeBeing on Metal 5 (fifth level), the capacitors have very small parasiticcapacitances to the substrate, simplifying matching of the ratioedcapacitors. The Metal-Insulator-Metal (MIM) structure of thesecapacitors results in low voltage and temperature coefficients andparasitic resistances.

Flash Microcontroller:

Having the 8-bit microcontroller 2 and the 64 KB Flash memory 9 on thesame chip as the sensors enables significant improvements in accuracyand functionality. This is because real-time continuous calibration orin-situ calibration over various conditions of temperature is achieved.This amount of memory is also sufficient to accommodate the entireIEEE802.15.4 protocol and Zigbee software stack to perform beacon,peer-to-peer, star and mesh networking, key requirements of modernwireless sensor networks. An on-chip regulator generates 1.2V, whichpowers most of the microcontroller, memory blocks, and wireless RFtransceiver, which are fabricated on thin-oxide minimum geometrydevices.

To facilitate lower power, the time-interval counter and part of themicrocontroller's interrupt logic are implemented on thick-oxide 3.3Vtransistors, as shown in FIG. 11. This means the regulator can beswitched off when the chip is in sleep or power-down mode, eliminatingthe DC bias currents of the regulator. This, together with almost-zerosub-threshold leakage of the 3V transistors, results in significantpower saving and elongation of battery life. On wakeup from power-down,the microcontroller also achieves reduction of noise and substratecrosstalk by operating the sensors, converters, and radio transceiversequentially.

Turning now to the wireless transceiver 3, and its low noise amplifier(LNA) in particular, the LNA is designed to have extra low power and lownoise operation. This is enabled by copper inductors on the fifth orsixth levels, and the use of strained silicon MOS devices for thefront-end LNA, see FIG. 12. This diagram shows a thin layer ofSilicon-Germanium 100, over which there is a thin strained silicon layer101, with higher carrier mobility than regular silicon. The polysilicongate 102 creates a channel in the strained silicon region. However themajority of the transistor current flows in the sub-surface SiGe region,due to the higher mobility of Germanium, giving lower noise operationand higher gain. The LNA can therefore be biased at lower currents forthe same gain, saving battery power. Copper has lower resistance thanaluminium, giving a higher Q-factor (resulting in higher receiver gain).The fifth or sixth level of copper is also thicker (lower resistance),and further away from the substrate (less parasitic capacitances).

Referring to FIG. 13, frequency selection for the RF transceiver 3 isshown. The device 1 forms a node in a wireless network of nodes. Thiscould be a simple point-to-point link or a star or mesh network. A fixedfrequency is used by all nodes and the wireless interface 3 provides aslow frequency hopping scheme to circumvent interferers. It operates byall nodes using the same frequency initially. Upon a transmissionfailure indicating possible interference, the nodes move to a differentfrequency according to an algorithm illustrated in FIG. 13. Therefollows synchronisation of all nodes.

All nodes are pre-programmed with the hop sequence for thefrequency-hopping scheme to work. Further, they must all be initialisedto the same channel so that they can “hop together”, typically afterinstallation or battery replacement.

In more detail, upon installation (or battery replacement), theinstaller manually puts the node into “initialise” mode, by, forexample, pressing a button. The node then switches on its receiver and“listens” for a nearby node transmission (or master beacon), on channel0 for example. If it receives nothing after an appropriate time, forexample a few seconds or minutes (because the current channel might beblocked), it steps to the next channel in the sequence, and again waitsand listens. Eventually by this means it should receive a beacon or datapacket from a neighbouring node; it can then re-synchronise its timer,request the hop interval timing, join the sequence, and go to sleepuntil the next hop and transmit period.

This initialisation method means the node has to stay “on” in full-powerreceive mode just once at installation; it can then revert to sleep modefor 99.9% of the time (as defined in the 802.15.4 standard) for the 1 to3 year lifetime of the battery. Since the 802.15.4 standard allows forsleep periods of up to about 4 minutes, the node could be in full-powerreceive mode for this duration. In practice this is unlikely, however,since the installer will know about this period. Using a spectrumanalyser (or hand-held wireless ‘sniffer’), he can roughly predict whenthe next beacon transmission is due, and press the ‘initialise’ buttonjust before this.

Referring to FIG. 14 this diagram shows an example of use of the slowhopping scheme. It is used on a long-distance (200 m) link 115 betweentwo buildings 120 and 125 (using a directional 14 dBi antenna on agateway node 126 linked with a computer 127). A standard 802.15.4 Zigbeefixed-channel star network of nodes 121 is implemented within the firstbuilding 120. This enables multi-vendor interoperable nodes to beinstalled in a star-network plant monitoring application, whereas theslow-hopping algorithm is employed on the long-distance critical link,which is more at risk of interference.

Testing and Calibration

This is traditionally difficult for humidity sensors, requiring specialchambers of controlled humidity, along with special package handling andelectrical connections.

In this invention, since the entire humidity sensor is fabricated in astandard CMOS process, it can be tested—and calibrated—at the normalwafer-level test before wafers are shipped. This takes advantage of thefact that wafer probe and factory test areas are generally operated at aprecise humidity level, for example 40% relative humidity. This knownvalue can be stored in on-chip Flash EEPROM memory for later use by themicrocontroller in correctly calibrating the output value under softwarecontrol, or it can be used in a non-Flash-EEPROM version of the chip toblow poly fuses to calibrate the sensor at 40% RH. This 1-pointcalibration may be sufficient for many applications, e.g. officeair-conditioning control around a setpoint, typically 40%. If moreaccuracy over a wider range of humidity is desired, then a secondcalibration point may be required. This is achieved by doing a“second-pass” wafer probe, in an enclosed chamber at 85% RH for example,or a dry-nitrogen dessicant chamber (0.001% RH). Although the secondpass wafer test adds some additional cost, it is significantly less thanpackage based testing.

Gas Sensing

In another embodiment, illustrated in FIG. 15, a thin film 130 of zincoxide and ferric oxide is deposited over passivation 131 at the locationof one of the differential capacitors 132 of the 18-bit Sigma-DeltaA-to-D converter 12. These oxides are synthesized by a sol-gel process,heated to about 120° C. to 200° C. then deposited by hybrid-ink-jetdeposition. The thin-film means that small finger spacings can be usedin the sensor structure, and the high-resolution A-to-D converter meansthat small sensor structures can be used and still result in detectableminute changes of capacitance, even at room temperature operation.

FIG. 16 shows an alternative embodiment, in whichferric-oxide/zinc-oxide 140 is deposited on top oxide or passivation141, but is connected directly to electrodes 142 in the top metallayers, forming a resistor whose value can be determined as part of abridge circuit by the 18-bit converter.

By use of different materials instead of the oxides 130 FIG. 15, thedevice architecture and production process may be adapted for sensingdifferent gases, such as using palladium for hydrogen sensing, Zirconiafor SO₂, H₂S, or Plasticised Polyvinyl chloride for NO₂, and WO₃ foriso-butane. In each case, both the conductivity and dielectric constantof the sensing material is changed by the ingressing gas, by adsorbtion,or physisorbtion, or chemisorbtion. Therefore the embodiments of15—capacitive—and 16—resistive—are used alternately or together inconjunction with the on-chip tightly integrated high resolutionconverter to achieve very low ppm gas concentration measurements.

Audio Sensors:

Alternatively, a piezo-electric polymer may be applied in theconfiguration shown in FIG. 16 for sound sensitivity. Transduction ispredominantly based on conductivity change. In this case, at the MOScircuit level a bridge circuit with buffer driving the 18-bit A-to-Dconverter is employed to capture the audio signal.

An audio sensor (microphone) is a useful feature on a remote wirelessnode, for example to “listen” if a motor is running, if an alarm bell isringing. Arrangements are needed for this audio due to the 0.1% dutycycle of IEEE802.15.4; the 250 Kb/s max data rate in the 802.15.4 2.4GHz band corresponds to a sustained constant data rate of 250 b/s at0.1% duty cycle. A variable-bit-rate audio compressor block (VBR) isemployed to achieve 15:1 or better compression ratio, achieving aneffective audio bit-rate of 3.75 Kb/s—sufficient for many industriallow-grade audio requirements.

Optical Sensors

Referring to FIG. 17 the device may also include an optical emitter 150and detector 151. Highly-directional deep anisotropic etching isemployed at the end of normal processing to fully etch away all six orseven layers of dielectric to expose a photodiode light sensor 151, alarge PN junction, 200 um×500 um, which collects photons and generates acorresponding electrical current.

The etch also reveals a porous silicon region 150 in this embodiment,created at the start of the process by electrochemical etching of thesubstrate in this particular region. Passing current through this makesit function as a light emitting diode (LED) due to the well knownluminescence property of porous silicon. Isolation trenches placedaround the porous region can minimize any currents leaking to thesubstrate and improve the light efficiency.

Electrochemical etching to form porous silicon is well known to thoseskilled in the art, and available on some CMOS processes, but isnon-standard on most CMOS processes. An alternative LED construction isa doped polymer organic light emitting device. Hybrid Ink-jet printingis used to directly deposit patterned luminescent doped-polymer films,for example polyvinylcarbazol (PVK) film, onto electrodes in the mannershown in FIG. 16.

The invention is not limited to the embodiments described but may bevaried in construction and detail. For example, conductors other thancopper may be used for the interconnects, such as aluminium. Also, thesensor device may be a “stripped down” version of the sensor, a“humidity-to-digital” sensor chip, having no radio or microcontroller orflash memory. In this case, calibration of the A-to-D and sensor isachieved by blowing various poly fuses in the voltage reference circuitand capacitor array. It should be noted that testing need not involvetesting every code of the A-to-D, thereby simplifying testingsignificantly, and reducing cost. Also, some or more of the followingfeatures may be provided individually or in combination in a method anddevice other than as described in the embodiments above:

-   -   use of strained silicon as a low noise amplifier,    -   low-frequency channel selection/hopping,    -   SAR with replication of the capacitor array,    -   porous silicon LED,    -   audio piezo-electric polymer microphone sensor,    -   audio compression and transmission at low duty cycle,    -   the microcontroller features.

1-37. (canceled)
 38. An integrated sensor device comprising: MOScircuits in a semiconductor substrate, an interconnect stack comprisinginterconnect levels having interconnect conductors and insulatingdielectric, said interconnect levels being over the substrate andinterconnecting the MOS circuits, a sensor including: electrodesembedded in the insulating dielectric and being integrally formed withthe interconnect conductors, and a porous material for ingress of gas orhumidity being sensed, and wherein the MOS circuits include an A-to-Dconverter for processing signals from the sensor electrodes, said A-to-Dconverter performing gas concentration measurements by detectingtogether changes of conductivity and dielectric constant of the sensorporous material.
 39. The integrated sensor device as claimed in claim38, wherein the sensor porous material includes an oxide film depositedover passivation, and the A-to-D converter includes differentialcapacitors.
 40. The integrated sensor device as claimed in claim 39,wherein the sensor porous material includes zinc oxide and ferric oxide.41. The integrated sensor device as claimed in claim 38, wherein thesensor porous material further includes a resistor interconnecting theelectrodes.
 42. The integrated sensor device as claimed in claim 41,wherein the resistor is of ferric oxide and zinc oxide composition. 43.The integrated sensor device as claimed in claim 39, wherein thepassivation layer is of Si₃N₄ composition.
 44. The integrated sensordevice as claimed in claim 38, wherein the MOS circuits include atemperature sensor.
 45. The integrated sensor device as claimed in claim38, wherein the MOS circuits include a temperature sensor, and whereinthe temperature sensor comprises a PNP transistor.
 46. The integratedsensor device as claimed in claim 38, wherein the MOS circuits include atemperature sensor, and wherein the MOS circuits include amicrocontroller for processing gas or humidity signals from the gas orhumidity sensor and temperature signals from the temperature sensor toprovide an enhanced output.
 47. The integrated sensor device as claimedin claim 46, wherein the enhanced output is temperature-corrected gas orhumidity readings.
 48. The integrated sensor device as claimed in claim38, wherein the A-to-D converter comprises an array of dummy capacitorswith a constant topography surrounding active A-to-D convertercapacitors.
 49. The integrated sensor device as claimed in claim 38,further comprising a light emitting diode.
 50. The integrated sensordevice as claimed in claim 49, wherein said diode is formed in a deeptrench to a lower interconnect level laterally of the sensor electrodes.51. The integrated sensor device as claimed in claim 38, wherein thedevice comprises a photo-detector diode.
 52. The integrated sensordevice as claimed in claim 51, wherein said diode is in a deep trench ina lower interconnect level laterally of the sensor electrodes.
 53. Theintegrated sensor device as claimed in claim 38, wherein the MOScircuits include a wireless transceiver.
 54. The integrated sensordevice as claimed in claim 53, wherein the wireless transceiver is forcommunication with other nodes in a network, and it comprises a meansfor switching channel frequency according to a low frequency channelswitching scheme upon detection of interference.
 55. The integratedsensor device as claimed in claim 38, wherein the device includes a lownoise amplifier.
 56. The integrated sensor device as claimed in claim38, wherein the device includes a low noise amplifier, and wherein thelow noise amplifier comprises a strained silicon MOS device beneath aconductor.
 57. A method of producing a sensor device comprising: MOScircuits in a semiconductor substrate, an interconnect stack comprisinginterconnect levels having interconnect conductors and insulatingdielectric, said interconnect levels being over the substrate andinterconnecting the MOS circuits, a sensor including: electrodesembedded in the insulating dielectric and being integrally formed withthe interconnect conductors, and a porous material for ingress of gas orhumidity being sensed, and wherein the MOS circuits include an A-to-Dconverter for processing signals from the sensor electrodes, said A-to-Dconverter performing gas concentration measurements by detectingtogether changes of conductivity and dielectric constant of the sensorporous material; the method comprising the steps of: fabricating the MOScircuits in the substrate, fabricating the interconnect levels insuccessive fabrication cycles according to interconnect design tointerconnect the MOS circuits, and fabricating the sensor electrodes anddielectric in a final interconnect level.
 58. The method as claimed inclaim 57, comprising the further step of depositing a passivation layerover a top interconnect level.
 59. The method as claimed in claim 57,comprising the further steps of depositing a passivation layer over atop interconnect level, depositing an etch stop layer over each layer ofdielectric in the interconnect levels, and depositing etch stop materialover the top interconnect level dielectric to provide the passivationlayer.
 60. The method of claim 57, wherein porous material is providedas a dielectric in lower interconnect levels and regular oxide is usedas a dielectric in upper interconnect levels.
 61. The method of claim57, wherein a strained low noise amplifier is deposited in the device,said amplifier comprising a strained silicon region.